The Type 12-Year Tail Fuze - U.S. Designation B-1(a) -
At right is a Diagram of the B-1(a) Fuze with Detonator, from the TM 9-1985-4 Army and Air Force 1953 training manual.
An instantaneous impact fuze used mostly on Japanese Army bomb types in the 100kg to 50kg class and likely to be found in conjunction with A-2 Series Nose fuzes.
This fuze shows an Osaka Arsenal mark and is dated March1943
The fuze body houses the striker and arming spindle. Vanes are screwed onto the the arming spindle which projects above the body.
When dropped, an arming wire is withdrawn and the vanes turn, and after twelve revolutions the vanes depart.
What remains is a simple impact striker, retained by a creep spring. There is no shear wire. On impact the striker is forced inward, compressing the creep spring and piercing the primer.